The magic mirror has awakened, but as its purpose isn't to tell who the most beautiful pony in Equestria is, Twilight's curiosity forces her to find out what its purpose really is before the reassuring knowledge of reality is shattered by shameful lies.

Or said with simpler yet more complicated terms:
"A mirror is a flat surface that reflects light. A magic mirror is the same thing, but enchanted, so it simply doesn't just reflect light. Fun fact, there are many magic mirrors in the world."